The three traditional ways of removing coatings from a composite material surface of an aircraft are sanding, scraping and chemical stripping. Sanding and scraping are manually done, very labor-intensive, and can cause surface gouges or abrasions. Chemical stripping using methylene chloride is the most efficient method, however, methylene chloride is a hazardous air pollutant and its use is scheduled to be eliminated by the year 1998 by the Clean Air Act. Sandblasting is another means of removing coatings from a surface. By impacting the coating with sand particles, sandblasting causes microfracture in the coating, allowing the coating to break free from the underlying surface. However, impacting off the coating with sand particles may cause some damage to the underlying composite material surface, such as scratches, pits, exposed fibers and even bond line damage. This makes sandblasting unacceptable for use on the types of aircraft which have most of the underlying surface made of composite material, or on a space shuttle orbiter which also has large areas of composite material.
It has been found that wheat starch, in the form of small crystallized particles, used in a blasting system, can effectively remove a coating from a composite material surface or an aluminum surface without damaging the surface. Due to the cutting characteristics of the sharp edges of the wheat starch particles, the coating is removed by being cut off the underlying surface instead of being impacted off the surface as in the case of sandblasting.
Currently, wheat starch media blasting is the only approved de-paint process for use on the types of aircraft which have the following coatings: polyurethane top coats, urethane primers and thick fluoro-elastomer rain-erosion coatings. The type of coating to be removed from certain portions of the surface of a space shuttle orbiter is silicone adhesive. Different types of coating require different angles of impingement and stand-off distances of blast media with respect to the surface to be treated. The more resilient the coating is, the lower the angle of impingement and the shorter the stand-off distance need to be. Thus, control of blast media angle of impingement and stand-off distance is a desired feature in a blasting system. Currently, this control is effected by a human operator in protective suit dispensing blast media from a nozzle attached to a blast hose. Since there is no containment and recapture of blast media, the de-paint process of an aircraft can only be done in a well-ventilated, environmentally controlled hangar, and not in an open environment. Presently, blasting systems called Vacuum Blasters, manufactured by VACU-BLAST Systems, Inc. and distributed by LTC Americas Inc. and others, provide containment and recapture of blast media, but provide practically no control of angle of impingement and stand-off distance. Thus, there is a need for a closed circuit media blasting system which provides control of blast media angle of impingement, stand-off distance, containment and recapture of blast media.